Roll sheet conveyance device and image forming apparatus incorporating same

ABSTRACT

An image forming apparatus includes an image forming device forming an image on a recording medium wound into a roll, and a roll sheet conveyance device unwinding and conveying the medium to the image forming device. The roll sheet conveyance device includes a roll sheet feed device, a torsion detector, and a control device. The roll sheet feed device includes a drive device including upstream and downstream shafts in a drive transmission direction to rotate the medium, and a feed device to feed the medium. The detection device includes a torsion coil spring provided between the upstream and downstream shafts, and a pair of an encoder and an encoder sensor detecting the torsion amount at the opposite ends of the torsion coil spring. The control device controls the torsion amount on the basis of a signal from the detection device, and applies a given back tension to the medium.

CROSS-REFERENCE TO RELATED APPLICATION

The present patent application claims priority pursuant to 35 U.S.C. §119 from Japanese patent application No. 2007-295298, filed on Nov. 14, 2007 in the Japan Patent Office, the contents and disclosures of which are hereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present patent application relates to a roll sheet conveyance device for unwinding and conveying a recording medium wound into a roll, and an image forming apparatus including the roll sheet conveyance device.

2. Discussion of the Background Art

Large-size electronic image forming apparatuses, such as copiers and printers that handle oversized documents such as drawings as well as other printing machines, perform an image forming operation by using a recording sheet (i.e., a recording medium) of a large size, e.g., the A0 size and the A1 size. To improve convenience in handling the large-size recording sheet in the image forming operation, such image forming apparatuses employ a mechanism that unwinds a roll sheet loaded therein as the recording sheet and conveys the roll sheet to an image forming unit.

A known roll sheet conveyance device includes a roll sheet supplying unit including a roll sheet unwinding shaft and a tensioner connected to the roll sheet unwinding shaft. The tensioner applies load torque to the roll sheet unwinding shaft to apply back tension to a roll sheet unwound from the roll sheet unwinding shaft. However, the load torque applied to the roll sheet unwinding shaft by the tensioner of the roll sheet supplying unit is constant, whereas the diameter of the roll of the roll sheet unwound from the roll sheet unwinding shaft is constantly reduced due to the consumption of the recording sheet. As a result, the back tension applied to the roll sheet by the tensioner is increased as the diameter of the roll is reduced over time by use of the roll sheet.

In view of the above, in one background technique, the load torque applied to the roll sheet unwinding shaft is reduced at a particular stage at which the diameter of the roll of the roll sheet unwound from the roll sheet unwinding shaft is reduced.

Such a background device includes a plurality of friction torque plates frictionally connected to the roll sheet unwinding shaft and lock devices for locking the respective friction torque plates. The background device unlocks one of the friction torque plates to reduce the load torque applied to the roll sheet unwinding shaft. Further, a roll sheet unwinding unit moves the roll sheet back and forth between an initial position and an unwinding position in every recording operation performed by a recording head, and a roll sheet supplying unit includes a roll sheet rewinding motor for rewinding the roll sheet around the roll sheet unwinding shaft. The roll sheet rewinding motor is connected via a unidirectional clutch to a lock shaft connected to the plurality of friction torque plates provided to the roll sheet unwinding shaft. The unidirectional clutch transmits to the lock shaft the reverse rotation of the roll sheet unwinding shaft caused by the roll sheet rewinding motor to rewind the roll sheet around the roll sheet unwinding shaft. Thereby, the rotation of the lock shaft, which accompanies the forward rotation of the roll sheet unwinding shaft for enabling the roll sheet to be unwound from the roll sheet unwinding shaft, is locked.

Further, an electromagnetic clutch is inserted between one of the plurality of friction torque plates and the lock shaft. If the diameter of the roll of the roll sheet exceeds a given value in a state in which the roll sheet is being unwound from the roll sheet unwinding shaft in the stop state of the roll sheet rewinding motor, the electromagnetic clutch is turned on to fixedly connect the friction torque plate to the lock shaft placed in the rotation locked state by the unidirectional clutch, and thus to lock the rotation of the friction torque plate. If the diameter of the roll of the roll sheet falls below a given value, the electromagnetic clutch is turned off to unlock the friction torque plate to allow free rotation. Further, in a state in which the roll sheet rewinding motor is driven to rewind the roll sheet around the roll sheet unwinding shaft, the electromagnetic clutch is in the ON state.

As described above, substantially constant back tension can be applied to the roll sheet irrespective of the diameter of the roll of the roll sheet. Further, even if oblique feeding occurs in the fed roll sheet, the roll sheet can be rewound for correction.

However, such a device uses a relatively large number of components including the plurality of friction torque plates, the unidirectional clutch, and the electromagnetic clutch. Thus, the background device is complicated and costly.

SUMMARY OF THE INVENTION

This patent specification describes a roll sheet conveyance device. In one example, a roll sheet conveyance device is configured to unwind and convey a recording medium wound into a roll to an image forming device configured to form an image on the recording medium. The roll sheet conveyance device includes a roll sheet feed device, a torsion detector, and a control device. The roll sheet feed device includes a drive device including an upstream shaft and a downstream shaft in a drive transmission direction to rotate the recording medium, and a feed device provided along a conveyance path of the recording medium to feed the recording medium. The torsion detector includes a torsion coil spring provided between the upstream shaft and the downstream shaft, and an encoder and an encoder sensor pair configured to detect the amount of torsion at the opposite ends of the torsion coil spring. The control device is configured to control the amount of torsion of the torsion coil spring on the basis of a signal output from the torsion detector, and to apply a given back tension to the recording medium.

This patent specification also describes an image forming apparatus. In one example, an image forming apparatus includes an image forming device configured to form an image on a recording medium wound into a roll, and a roll sheet conveyance device configured to unwind and convey the recording medium to the image forming device. The roll sheet conveyance device includes a roll sheet feed device, a torsion detector, and a control device. The roll sheet feed device includes a drive device including an upstream shaft and a downstream shaft in a drive transmission direction to rotate the recording medium, and a feed device provided along a conveyance path of the recording medium to feed the recording medium. The torsion detector includes a torsion coil spring provided between the upstream shaft and the downstream shaft, and an encoder and an encoder sensor pair configured to detect the amount of torsion at the opposite ends of the torsion coil spring. The control device is configured to control the amount of torsion of the torsion coil spring on the basis of a signal output from the torsion detector, and to apply a given back tension to the recording medium.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the advantages thereof are obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a configuration of an inkjet printer illustrating an embodiment of an image forming apparatus according to the present patent application;

FIG. 2 is a perspective view illustrating one side of a roll sheet feed tray forming a roll sheet conveyance device;

FIG. 3 is a cross-sectional explanatory view of a torsion mechanism;

FIGS. 4A and 4B are explanatory graphs illustrating the relation between rotation angles of drive shafts and back tension in uniform speed conveyance without an initial torsion angle;

FIG. 5 is a flowchart illustrating control of roll sheet conveyance in the uniform speed conveyance;

FIGS. 6A and 6B are explanatory graphs illustrating the relation between the rotation angles of the drive shafts and the back tension in uniform speed conveyance with the initial torsion angle;

FIGS. 7A and 7B are explanatory graphs illustrating the relation between the rotation angles of the drive shafts and the back tension in intermittent conveyance without the initial torsion angle;

FIG. 8 is a flowchart illustrating control of the roll sheet conveyance in the intermittent conveyance;

FIGS. 9A and 9B are explanatory graphs illustrating the relation between the rotation angles of the drive shafts and the back tension in intermittent conveyance with the initial torsion angle;

FIGS. 10A and 10B are explanatory graphs illustrating the relation between the rotation angles of the drive shafts and the back tension in a modified example of the intermittent conveyance with the initial torsion angle;

FIG. 11 is a flowchart illustrating control to check oblique feeding in the setting of a roll sheet;

FIG. 12 is a graph illustrating the relation between amount of oblique feeding to be corrected and magnitude of the back tension;

FIG. 13 is a perspective view illustrating a modified configuration of the torsion mechanism; and

FIG. 14 is a perspective view illustrating a modified configuration of a torsion detector.

DETAILED DESCRIPTION OF THE INVENTION

In describing the embodiments illustrated in the drawings, specific terminology is employed for the purpose of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so used, and it is to be understood that substitutions for each specific element, can include any technical equivalents that operate in a similar manner and achieve a similar result.

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to FIG. 1, an embodiment of the present patent application will be described in detail.

FIG. 1 is a schematic configuration diagram of an inkjet printer 100 illustrating an embodiment of an image forming apparatus according to the present patent application. In FIG. 1, the reference numeral 1 designates an apparatus body of the inkjet printer 100. The apparatus body 1 includes an image scanning unit 30 provided in an upper part thereof, an image forming unit 2 provided below the image scanning unit 30, and a sheet feeding unit 50 provided below the image forming unit 2.

The image scanning unit 30 includes a document table 31, a contact image sensor (CIS) 32, a sheet discharge tray 33, sheet feeding rollers 34, conveying rollers 35, and sheet discharging rollers 36. The image scanning unit 30 is configured to feed a document placed on the document table 31 to a scanning position at the contact image sensor 32, scan the image of the document through the contact image sensor 32, and discharge the document onto the sheet discharge tray 33. The document, placed on the document table 31 is aligned in the width direction by side fences (not illustrated) and fed by the sheet feeding rollers 34 to be conveyed to a position under the contact image sensor 32. On the document table 31, a document width detection sensor and a document length detection sensor (not illustrated) are provided to detect the size of the document conveyed from the document table 31. The document set under the contact image sensor 32 is exposed to light emitted from a light source (not illustrated) such as an LED (Light-Emitting Diode) array and a fluorescent lamp. Then, resultant reflected light is transmitted through a rod lens array (not illustrated), formed into an image on the contact image sensor 32, and subjected to photoelectric conversion. The scanned document is discharged onto the sheet discharge tray 33 by the conveying rollers 35 and the sheet discharging rollers 36.

The image forming unit 2 includes an imaging unit 3 including a carriage 6 mounting an inkjet head (not illustrated), a suction conveying unit 4 including a suction device 7 and a conveyor belt 8, and a sheet discharge unit 5. The image forming unit 2 further includes sheet discharge trays 9 and 10, registration rollers 11, a sheet discharge and sorting mechanism 12, a cutter unit 13, and sheet discharge and conveyance rollers 14, 15, and 16. In the image forming unit 2, an image signal scanned by the contact image sensor 32 is subjected to image processing. Meanwhile, the inkjet head mounted on the carriage 6 discharges liquid ink droplets onto a recording sheet (i.e., a recording medium) to form an image. The method of discharging the ink from the inkjet head is not fixed and includes a piezoelectric method of discharging the ink by using a piezoelectric effect as well as a bubble jet method using a technique of discharging boiled liquid droplets. The present embodiment can use either one of the methods.

The recording sheet transferred with the image of the document by the image forming unit 2 is conveyed through the sheet discharge unit 5 to be discharged onto the sheet discharge tray 9 forming an upper surface of the image forming unit 2 or the sheet discharge tray 10 located rearward of the imaging unit 3 (i.e., to the right in FIG. 1).

The sheet feeding unit 50 for feeding the recording sheet to the image forming unit 2 includes a roll sheet feed tray 51 and a cut sheet feed tray 52. The roll sheet feed tray 51 is configured to be slidable from the apparatus body 1 toward the left in FIG. 1, and to enable the process of setting or unjamming a roll sheet to be performed in the opened state of the roll sheet feed tray 51. Similarly, the cut sheet feed tray 52 is also configured to be slidably detachable from the apparatus body 1 toward the left in FIG. 1, and to enable the process of setting or unjamming a cut sheet to be performed in the opened state of the cut sheet feed tray 52.

The roll sheet feed tray 51 is capable of setting therein two roll sheets 60 and 61. The reference numerals 62 and 63 in FIG. 1 designate a sheet holder and a roll sheet rest, respectively. Meanwhile, the cut sheet feed tray 52 has a bi-level structure to enable recording sheets of a variety of standard sizes to be set therein.

The recording sheet fed from the roll sheet feed tray 51 or the cut sheet feed tray 52 is conveyed into the image forming unit 2 by the registration rollers 11 in synchronization with image forming timing. The recording sheet conveyed into the image forming unit 2 is held and conveyed by the conveyor belt device 8 of the suction conveying unit 4, which has a suction and absorption function. To enable the liquid ink droplets discharged from the inkjet head of the imaging unit 3 to be discharged at the appropriate position, the suction conveying unit 4 is provided substantially parallel to the carriage 6 mounting the inkjet head.

The inkjet image forming method includes a method in which the inkjet head performs a scanning operation in the main scanning direction to discharge the liquid ink droplets and form an image as well as a method in which the inkjet head fixed within a range of the corresponding sheet width discharges the liquid ink droplets and form an image. In the former method, it is desirable to perform intermittent conveyance of repeating the drive and stop of the recording sheet in the image forming unit 2. By contrast, in the latter method, it is desirable to perform uniform speed conveyance of conveying the recording sheet at a uniform speed in the image forming unit 2.

The recording sheet formed with the image is conveyed to the sheet discharge unit 5 by the suction conveying unit 4. Then, the recording sheet is sorted by the sheet discharge and sorting mechanism 12 and conveyed to either one of the sheet discharge trays 9 and 10 selected by a user. If the roll sheet is used as the recording sheet, the sheet discharge and conveyance rollers 14, 15, and 16 are stopped when the distance from the cutter unit 13 to the leading end of the recording sheet reaches a given length. Then, the cutter unit 13 is activated to cut the recording sheet. Thereafter, the sheet discharge and conveyance rollers 14, 15, and 16 are activated to convey the cut recording sheet to either one of the sheet discharge trays 9 and 10 to stack the recording sheet thereon.

With reference to FIG. 2, a description will now be given of the configuration of a roll sheet conveyance device according to an embodiment of the present patent application.

FIG. 2 is a perspective view illustrating one side of the roll sheet feed tray 51 forming the roll sheet conveyance device. FIG. 2 illustrates the roll sheet 60, a pair of the sheet holders 62, the roll sheet rest 63, a drive motor 64, a worm gear 65, a worm wheel 66, a drive gear 67, a drive transmission rod 68, a drive shaft 69, a torsion mechanism 70 (FIG. 3 is a cross-sectional explanatory view of the torsion mechanism), sheet encoders 72 and 73, encoder sensors 74 and 75, conveying rollers 80 provided with a shaft 80 a, conveying rollers 81, a conveyance drive motor 82, a gear 83, a drive gear 84, a drive transmission belt 85, a sheet encoder 86, an encoder sensor 87, and a sheet end surface position detection sensor 88. The roll sheet 60 wound into a roll is placed on the roll sheet rest 63, with opposed ends of the roll sheet 60 held by the respective sheet holders 62.

It is desirable to configure the roll sheet rest 63 so as to minimize the rotational resistance of the sheet holders 62. Such a configuration includes a configuration in which an outer circumferential portion of each of the sheet holders 62 is in contact with a corresponding ball bearing provided to a shaft included in the roll sheet rest 63. The leading end of the thus-placed roll sheet 60 engages the conveying rollers 80 and 81, which is driven by a configuration different from a roll sheet feeding mechanism, and is conveyed to a conveyance path downstream of the conveying rollers 80 and 81.

One of the sheet holders 62 is connected to the drive shaft 69. The rotational drive of the drive motor 64 is transmitted to the drive shaft 60 via the worm gear 65, the worm wheel 66, the drive gear 67, and the drive transmission rod 68. Thereby, the roll sheet 60 can be driven to rotate. That is, the drive transmission rod 68 and the drive shaft 69 serve as an upstream shaft and a downstream shaft, respectively, in the drive transmission direction. Between the drive transmission rod 68 and the drive shaft 69, the torsion mechanism 70 is provided which includes a component having a spring constant, i.e., a torsion coil spring 71 (see FIG. 3). When the roll sheet 60 is pulled toward the conveying rollers 80, the torsion coil spring 71 is twisted to generate a resilient force (resilience). The thus-generated resilience is used as back tension for conveying the roll sheet 60.

To detect the amount of torsion at the opposite ends of the torsion mechanism 70, a torsion detector is provided. The torsion detector of the present embodiment includes the sheet encoders 72 and 73 and the encoder sensors 74 and 75. The sheet encoders 72 and 73 are fixed to the drive transmission rod 68 and the drive shaft 69, respectively. The encoder sensors 74 and 75 detect the rotation of the sheet encoder 72 and the rotation of the sheet encoder 73, respectively. On the basis of the rotation of the sheet encoder 72 and the rotation of the sheet encoder 73 detected by the encoder sensors 74 and 75, respectively, the respective angular velocities of the drive transmission rod 68 and the drive shaft 69 can be known.

Herein, it is desirable that the sheet encoders 72 and 73 have the same configuration or the same number of lines around the entire circumference thereof. The resilience of the torsion coil spring 71 is increased in direct proportion to the amount of torsion of the drive transmission rod 68 and the drive shaft 69. Therefore, if the difference in counts between the sheet encoders 72 and 73 is set, the back tension can be controlled.

As illustrated in FIG. 3, the torsion mechanism 70 includes a twist member 76 fixedly connected to one end of the drive transmission rod 68 and a twist member 77 fixedly connected to one end of the drive shat 69. With the twist members 76 and 77 combined together to be rotatably engaged, a hollow cylindrical space is formed therein. The thus-formed hollow cylindrical space includes the torsion coil spring 71, the opposite ends of which are fixed to the twist members 76 and 77, respectively.

In the thus-configured torsion mechanism 70, if the drive shaft 69 is forced to rotate while the drive transmission rod 68 is fixed, a resilience is obtained that is determined by the spring constant of the torsion coil spring 71 and the rotation angle. If the drive transmission rod 68 and the drive shaft 69 are rotated at an equal angular velocity in a state in which a given intended resilience has been obtained, the drive transmission rod 68 and the drive shaft 69 can be rotated while the given intended resilience is maintained.

The conveying rollers 80 are provided with the shaft 80 a, one end of which is provided with the drive gear 84 meshing with the gear 83 attached to the output shaft of the conveyance drive motor 82. The drive of the conveying rollers 80 is transmitted to the conveying rollers 81 via the drive transmission belt 85. The conveying rollers 80 and 81 are driven by the conveyance drive motor 82, which is a drive source different from a drive source for driving the roll sheet 60. The shaft 80 a of the conveying rollers 80 is provided with the sheet encoder 86 and the encoder sensor 87 for detecting the rotation of the sheet encoder 86.

In FIG. 2, the sheet end surface position detection sensor 88 can detect the occurrence or non-occurrence of oblique feeding of the roll sheet 60. Further, FIG. 2 omits illustration of driven rollers paired with the conveying rollers 80 and 81.

The roll sheet conveyance device of the present embodiment includes a control device, not illustrated, whose operation is described below.

FIGS. 4A and 4B are explanatory graphs illustrating the relation between the rotation angles of the drive shafts (i.e., the drive transmission rod 68 and the drive shaft 69) and the back tension in uniform speed conveyance without an initial torsion angle. Further, FIG. 5 is a flowchart illustrating control of the roll sheet conveyance performed in the uniform speed conveyance.

In FIG. 5, upon issuance of a command to perform the uniform speed conveyance of the roll sheet 60, whether or not the torsion angle of the drive transmission rod 68 and the torsion angle of the drive shaft 69 have reached the intended value is determined on the basis of respective signals output from the encoder sensors 74 and 75 (step S1). If the intended torsion angle value has been obtained, the conveyance of the conveying rollers 80 and 81 and the unwinding of the roll sheet 60 are driven at a uniform speed while the intended torsion angle is maintained (step S2). By contrast, if it is determined at Step 1 that the intended torsion angle has not been obtained, the drive motor 64 is stopped (step S3), and the conveying rollers 80 are driven to unwind the roll sheet 60 (step S4). Then, whether or not the torsion angle has reached the intended value is again determined at step S5. If the torsion angle has reached the intended torsion angle value, the drive motor 64 is driven to perform the conveyance of the conveying rollers 80 and 81 and the unwinding of the roll sheet 60 at a uniform speed (step S6). That is, in FIG. 4A in which the drive shaft 69 and the drive transmission rod 68 are represented by a broken line and a solid line, respectively, when the drive shaft 69 is rotated and the torsion angle thereof has reached the intended torsion angle, which is equal between the drive shaft 69 and the drive transmission rod 68, the drive transmission rod 68 is also driven to rotate by the drive motor 64 to maintain the intended torsion angle.

With the above-described control, constant back tension as illustrated in FIG. 4B can be obtained. Similar control is performed also in uniform speed conveyance with the initial torsion angle. In such a case, the time taken to obtain the intended torsion angle is reduced, as observed in FIG. 6B.

FIGS. 7A and 7B are explanatory graphs illustrating the relation between the rotation angles of the drive shafts (i.e., the drive transmission rod 68 and the drive shaft 69) and the back tension in intermittent conveyance without the initial torsion angle. FIG. 8 is a flowchart illustrating control of the roll sheet conveyance performed in the intermittent conveyance.

In FIG. 8, upon issuance of a command to perform the intermittent conveyance of the roll sheet 60, whether or not the torsion angle of the drive transmission rod 68 and the torsion angle of the drive shaft 69 have reached the intended value is determined on the basis of respective signals output from the encoder sensors 74 and 75 (step S1). If the intended torsion angle value has been obtained, the intermittent conveyance is continued in which the conveyance of the conveying rollers 80 and 81 and the unwinding of the roll sheet 60 are performed in synchronization with each other while the intended torsion angle is maintained (step S2). By contrast, if it is determined at Step 1 that the intended torsion angle has not been obtained, the drive motor 64 is stopped.(step S3), and the conveying rollers 80 are driven to unwind the roll sheet 60 and perform the intermittent conveyance of the roll sheet 60 (step S4). Then, whether or not the torsion angle has reached the intended value is again determined at step S5. If the torsion angle has reached the intended torsion angle value, the drive motor 64 is driven to perform the intermittent conveyance in which the conveyance of the conveying rollers 80 and 81 and the unwinding of the roll sheet 60 are performed in synchronization with each other (step S6). That is, in FIG. 7A, in which the drive shaft 69 and the drive transmission rod 68 are represented by a broken line and a solid line, respectively, the intermittent conveyance is performed such that, when the drive shaft 69 is rotated and the torsion angle thereof reaches the intended torsion angle, which is equal between the drive shaft 69 and the drive transmission rod 68, the drive transmission rod 68 is also driven to rotate by the drive motor 64 to maintain the intended torsion angle.

With the above-described control, constant back tension as illustrated in FIG. 7B can be obtained also in the intermittent conveyance. Similar control is performed also in intermittent conveyance with the initial torsion angle. In such a case, the time taken to obtain the intended torsion angle is reduced, as observed in FIG. 9B. Further, in the intermittent conveyance with the initial torsion angle, it is also possible to perform the control of the roll sheet conveyance by performing uniform speed driving of the drive motor 64 instead of intermittent driving of the drive motor 64. In this case, if the unwinding speed of the roll sheet 60 based on the uniform speed driving of the drive motor 64 is set to be slightly slower than the conveyance speed of the intermittently driven conveying rollers 80, the back tension applied to the roll sheet 60 can be maintained substantially constant, although slight waves are present as shown in FIG. 10B.

Meanwhile, if the amount of oblique conveyance (i.e., oblique feeding) occurring in the conveyed roll sheet 60 exceeds a specified value, image quality deteriorates substantially. Oblique feeding is caused by defective setting of the roll sheet 60. Therefore, it is necessary to detect the occurrence or non-occurrence of oblique feeding in the set roll sheet 60, and to reset the roll sheet 60 in the event of oblique feeding.

In view of the above, control as illustrated in FIG. 11 is performed in the setting of the roll sheet 60 in the present embodiment. Firstly, the leading end of the roll sheet 60 is conveyed to the position of the sheet end surface position detection sensor 88 (step S1), and the sheet end surface position detection sensor 88 detects the amount of oblique feeding (step S2). Then, whether or not the detected amount of oblique feeding is equal to or less than a specified value is determined (step S3). If the detected amount of oblique feeding is equal to or less than the specified value, the setting of the roll sheet 60 is completed. Meanwhile, if the detected amount of oblique feeding exceeds the specified value, the drive motor 64 is reversely rotated to rewind the roll sheet 60. Herein, the driving of the conveying rollers 80 is controlled to rewind the roll sheet 60 while maintaining the back tension. Thereby, the roll sheet 60 is rewound into the roll without having a twist or the like generated therein (step S4). Then, the rewound roll sheet 60 is reset (step S5), and the occurrence or non-occurrence of oblique feeding is again checked.

The above-described control can substantially prevent oblique feeding of the set roll sheet 60. If the amount of oblique feeding of the set roll sheet 60 is relatively large, the roll sheet 60 may be rewound without being corrected by the given back tension. The amount of oblique feeding to be corrected and the magnitude of the back tension are proportional, as illustrated in FIG. 12. Thus, it is preferable to set the magnitude of the back tension in accordance with the amount of oblique feeding. That is, if the amount of oblique feeding is increased, the back tension is also increased. In this case, the conveyance of the conveying rollers 80 is controlled on the basis of a signal output from the encoder sensor 87, which detects the rotation of the sheet encoder 86.

In the preferred embodiment of the present patent application described above, if some kind of accident or the like causes erroneous control and resultant application of very large back tension to the roll sheet 60, not only damage to the roll sheet 60 but also damage to the conveyance device may occur.

Accordingly, the torsion mechanism 70 may be modified as illustrated in FIG. 13, wherein the twist member 76 includes a projection 76 a and the twist member 77 includes a groove 77 a for controlling the range in which the projection 76 a can move. With this configuration, it is possible to set upper and lower limits on the amount of torsion of the torsion mechanism 70.

Further, to detect the amount of torsion, the above-described embodiment includes the sheet encoders 72 and 73 fixed to the drive transmission rod 68 (the upstream shaft) and the drive shaft 69 (the downstream shaft), respectively, and the encoder sensors 74 and 75 for detecting the rotation of the sheet encoder 72 and the rotation of the sheet encoder 73, respectively. Alternatively, the torsion detector may be modified as illustrated in FIG. 14, wherein the encoder sensor 74 is formed by a bracket 68 a fixed to the drive transmission rod 68 to detect the rotation of the sheet encoder 72 fixed to the drive shaft 69.

With this configuration, the amount of torsion can be detected with the use of a single pair of the sheet encoder 72 and the encoder sensor 74.

The above-described embodiments are illustrative and do not limit the present patent application. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements at least one of features of different illustrative and exemplary embodiments herein may be combined with each other at least one of substituted for each other within the scope of this disclosure and appended claims. Further, features of components of the embodiments, such as the number, the position, and the shape, are not limited to the embodiments described and thus may be set as preferred. It is therefore to be understood that, within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. 

1. An image forming apparatus comprising: an image forming device configured to form an image on a recording medium wound into a roll; and a roll sheet conveyance device configured to unwind and convey the recording medium to the image forming device, the roll sheet conveyance device including: a roll sheet feed device including a drive device including an upstream shaft and a downstream shaft in a drive transmission direction to rotate the recording medium, and a feed device provided along a conveyance path of the recording medium to feed the recording medium; a torsion detector including a torsion coil spring provided between the upstream shaft and the downstream shaft, and an encoder and an encoder sensor pair configured to detect an amount of torsion at opposite ends of the torsion coil spring; and a control device configured to control the amount of torsion of the torsion coil spring on the basis of a signal output from the torsion detector and apply a given back tension to the recording medium.
 2. The image forming apparatus as described in claim 1, wherein the encoder of the torsion detector is provided to each of the upstream shaft and the downstream shaft, and faces the corresponding encoder sensor fixed thereto.
 3. The image forming apparatus as described in claim 1, wherein the encoder of the torsion detector is provided to either one of the upstream shaft and the downstream shaft, and the corresponding encoder sensor is provided to the other one of the upstream shaft and the downstream shaft.
 4. The image forming apparatus as described in claim 1, wherein, upon detection of oblique feeding of the recording medium, the control device rewinds the recording medium while maintaining the given back tension.
 5. The image forming apparatus as described in claim 4, wherein, if an amount of oblique feeding of the recording medium exceeds a given amount, the control device sets the amount of torsion of the torsion coil spring to be larger in reverse rotation for rewinding the recording medium than in forward rotation for feeding the recording medium.
 6. An image forming apparatus comprising: image forming means for forming an image on a recording medium wound into a roll; and roll sheet conveying means for unwinding and conveying the recording medium to the image forming means, the roll sheet conveying means including: roll sheet feeding means including drive means for rotating the recording medium, and feeding means for feeding the recording medium; torsion amount detection means including torsion resilience means for generating resilience, and encoder and sensor means for detecting an amount of torsion at opposite ends of the torsion resilient means; and control means for controlling the amount of torsion of the torsion resilient means on the basis of a signal output from the torsion amount detection means, and applying a given back tension to the recording medium.
 7. A roll sheet conveyance device configured to unwind and convey a recording medium wound into a roll to an image forming device configured to form an image on the recording medium, the roll sheet conveyance device comprising: a roll sheet feed device including a drive device including an upstream shaft and a downstream shaft in a drive transmission direction to rotate the recording medium, and a feed device provided along a conveyance path of the recording medium to feed the recording medium; a torsion detector device including a torsion coil spring provided between the upstream shaft and the downstream shaft, and an encoder and an encoder sensor pair configured to detect an amount of torsion at the opposite ends of the torsion coil spring; and a control device configured to control the amount of torsion of the torsion coil spring on the basis of a signal output from the torsion detector and apply a given back tension to the recording medium.
 8. The roll sheet conveyance device as described in claim 7, wherein the encoder of the torsion detector is provided to each of the upstream shaft and the downstream shaft, and faces the corresponding encoder sensor fixed thereto.
 9. The roll sheet conveyance device as described in claim 7, wherein the encoder of the torsion detector is provided to either one of the upstream shaft and the downstream shaft, and the corresponding encoder sensor is provided to the other one of the upstream shaft and the downstream shaft.
 10. The roll sheet conveyance device as described in claim 7, wherein, upon detection of oblique feeding of the recording medium, the control device rewinds the recording medium while maintaining the given back tension.
 11. The roll sheet conveyance device as described in claim 10, wherein, if an amount of oblique feeding of the recording medium exceeds a given amount, the control device sets the amount of torsion of the torsion coil spring to be larger in reverse rotation for rewinding the recording medium than in forward rotation for feeding the recording medium. 